Re: valgrind warnings [Re: Emacs bzr memory footprint]

[Eli Zaretskii <eliz@gnu.org>]

> From: Dan Nicolaescu <dann@gnu.org>
> Cc: "Stephen J. Turnbull" <stephen@xemacs.org>,  emacs-devel@gnu.org
> Date: Fri, 28 Oct 2011 10:38:03 -0400
> 
> > Valgrind produces these warnings, maybe some of them are interesting:
> >
> > ==30382== Conditional jump or move depends on uninitialised value(s)
> > ==30382==    at 0x4731E6: note_mouse_highlight (xdisp.c:26841)
> > ==30382==    by 0x4F8682: note_mouse_movement (xterm.c:3830)
> > ==30382==    by 0x4FCB2E: handle_one_xevent (xterm.c:6652)
> > ==30382==    by 0x4FD792: XTread_socket (xterm.c:7133)
> > ==30382==    by 0x5437F7: read_avail_input (keyboard.c:6821)
> > ==30382==    by 0x54409F: handle_async_input (keyboard.c:7149)
> > ==30382==    by 0x5440BE: process_pending_signals (keyboard.c:7165)
> > ==30382==    by 0x62D713: wait_reading_process_output (process.c:4340)
> > ==30382==    by 0x419ED2: sit_for (dispnew.c:5971)
> > ==30382==    by 0x53AF87: read_char (keyboard.c:2687)
> > ==30382==    by 0x548876: read_key_sequence (keyboard.c:9290)
> > ==30382==    by 0x538709: command_loop_1 (keyboard.c:1447)
> 
> This is because window_from_coordinates in note_mouse_highlight is
> passed "&part", where "part" is an uninitialized local variable.
> When !NILP(window) "part" is not touched, so it stays uninitialized.

That's not my reading of the code.  Here's what happens in
window_from_coordinates:

  window = Qnil;
  cw.window = &window, cw.x = x, cw.y = y; cw.part = part;
  foreach_window (f, check_window_containing, &cw);

So cw.part now holds "part", which is a pointer.
check_window_containing does this:

  found = coordinates_in_window (w, cw->x, cw->y);
  if (found != ON_NOTHING)
    {
      *cw->part = found;
      XSETWINDOW (*cw->window, w);

So if it finds a window, it touches "part" through cw->part and sets
`window' (through cw->window) to a non-nil value.  Now back in
window_from_coordinates:

  if (NILP (window)
      && tool_bar_p
      && WINDOWP (f->tool_bar_window)
      && WINDOW_TOTAL_LINES (XWINDOW (f->tool_bar_window)) > 0
      && (coordinates_in_window (XWINDOW (f->tool_bar_window), x, y)
	  != ON_NOTHING))
    {
      *part = ON_TEXT;
      window = f->tool_bar_window;
    }

If `window' is non-nil, "part" was already set inside
check_window_containing, so there's no need to set it again.

Am I missing something or did this tricky code dupe valgrind?